Hum, there must be a bug due to oversimplification in the website software then... My reasoning for that is:
a) as you say, if I look at the ProPilot division under Fleet there are many B737-800 listed as operational, so apparently they do exist as you confirmed.
b) Looking in the Lock Flight menu of ProPilot and enter as ONLY search criteria
- Search by Airplane Class
- checked Class 5 (all other classes unchecked)
so this should search for "all class 5 aircraft on ProPilot regardless of their location".
The result shown there after executing the search shows many Airbuses and only one B737-800.
As it turns out you can choose the length of the page (default is about 40), but if I choose 400 then I see all the boeings.
I think the one that programmed the website forgot to include the paging mechanism that is common nowadays where you can select items per page and then navigate through the pages.
Anyway, I see it now but IMHO that search form can be enhanced

Thanks